Systems and Methods for Extending Gas Turbine Emissions Compliance

ABSTRACT

A gas turbine system for operation at low loads. The gas turbine system may include a number of inlet guide vanes, a compressor, a turbine, and an air movement system for maintaining an emission from the gas turbine system below a predetermined level.

TECHNICAL FIELD

The present application relates generally to gas turbines and more particularly relates to methods and systems for extending gas turbine emissions compliance at lower loads.

BACKGROUND OF THE INVENTION

Due to rising fuel costs, natural gas fired power plants that were designed to operate at mostly full power output are now being operated on a intermittent basis. Coal and nuclear energy now generally make up the majority of stable power output. Gas turbines are being increasingly used to make up the difference during peak demand periods. For example, a gas turbine may be used only during the daytime and then taken off line during the nighttime when the power demand is lower.

During load reductions or “turndowns”, gas turbines typically can remain in emissions compliance down to about forty-five percent (45%) of full rated load output. Below this load, carbon monoxide (CO) emissions can increase exponentially and cause the system as a whole to go out of emissions compliance. Generally described, emissions compliance requires that the turbine as a whole to produce less than the guaranteed or predetermined minimum emissions levels. Such levels may vary with the ambient temperature, system size, and other variables.

If a gas turbine has to be shutdown because it cannot remain in emissions compliance due to a low power demand, the other equipment in a combined cycle application also may need to be taken offline. This equipment may include a heat recovery steam generator, a steam turbine, and other devices. Bringing these other systems online again after a gas turbine shutdown may be expensive and time consuming.

Such startup requirements may prevent a power plant from being available to produce power when the demand is high. There may be a strategic operational advantage in being able to keep a gas turbine online and in emissions compliance during periods of low power demand so as to avoid the start up time and expense.

There is a desire therefore for methods for extending gas turbine emissions compliance during periods of reduced loads. Reducing the load on the gas turbine while remaining in emissions compliance may enable the operator to take advantage of these peak demand opportunities.

SUMMARY OF THE INVENTION

The present application thus provides a gas turbine system for operation at low loads. The gas turbine system may include a number of inlet guide vanes, a compressor, a turbine, and an air movement system for maintaining an emission from the gas turbine system below a predetermined level.

The present application further describes a gas turbine for operation at low loads. The gas turbine may include a number of inlet guide vanes, a compressor, and an air recirculation system to raise a temperature of an outlet air stream leaving the compressor.

The present application further describes a gas turbine system for operation at low loads. The gas turbine system may include a number of inlet guide vanes, a compressor, a turbine, and an air extraction system to extract air from the compressor.

These and other features of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an inlet bleed heat configuration.

FIG. 2 is a schematic view of a compressor recirculation configuration.

FIG. 3 is a schematic view of a compressor extraction configuration.

FIG. 4 is a schematic view of a compressor discharge casing configuration.

DETAILED DESCRIPTION

Referring now to the drawings, in which like numerals refer to like elements throughout the several views, FIG. 1 is a schematic view of a gas turbine system 100. The gas turbine system 100 may include a compressor 110 with a compressor discharge casing 120, a combustor 130, and a turbine 140. Generally described, the gas turbine system 100 receives ambient air through a set of inlet guide vanes 150. The ambient air is compressed by the compressor 110 and delivered to the combustor 130 where it is used to combust a flow of fuel to produce a hot combustion gas. The hot combustion gas is delivered to the turbine 140 where it is expanded to mechanical energy via a number of blades and a shaft. The turbine 140 and the compressor 110 are generally connected to a common shaft that also may be connected to an electric generator or other type of load. Extending emissions compliance may be possible by raising the combustion reaction zone temperatures to inhibit CO (carbon monoxide) formation and also to provide flame stability. Emissions compliance means that the emissions from the gas turbine system 100 as a whole are maintained below predetermined levels.

A first technique involves the use of inlet bleed heat and reducing the angles for the inlet guide vanes 150. Reducing the minimum angles for the inlet guide vane 150 reduces the core airflow through the gas turbine system 100 so as to raise the reaction zone temperature in the combustor 130. During a turndown, the angles of the inlet guide vanes 150 may be reduced until the minimum angle or an exhaust temperature isotherm is reached. Operation above this temperature level may cause damage to downstream components. After reaching either of these limits, a decrease in the load requires a reduction in the fuel flow. This reduction, however, may decrease the reaction zone temperature in the combustor 130 and may promote CO formation. A further reduction in the minimum angle for the inlet guide vanes 150 therefore may allow operation along the exhaust temperature isotherm at a lower load before a reduction in fuel flow may be needed. These minimum angles may result in an improved turndown over a portion of the ambient temperature range. Depending on the nature of the gas turbine 100, angles of about 30 to about 50 degrees may be used herein, with a typical full operating range extending from about 40 to about 90 degrees. Other angles may be used herein.

The angles of the inlet guide vanes 150 generally are opened to maintain exhaust temperatures at or below the isotherm. Increasing the exhaust temperature isotherm also may permit operation at lower angles of the inlet guide vanes 150. Increasing the isotherm may be accomplished by adjusting the operating parameters of the gas turbine 100 as a whole. Further, variations in the isotherm may be caused by adding duct insulation, different material selection, and varying other components.

In the embodiment of FIG. 1, an inlet bleed heat configuration 155 is shown. This configuration includes an inlet bleed heat line 160 that may be positioned between the compressor discharge casing 120 and the inlet guide vanes 150. The inlet bleed heat line 160 extracts air from the compressor discharge casing 120 and introduces it upstage of the inlet guide vanes 150. An inlet bleed heat line valve 170 may be positioned thereon. The valve 170 may be of conventional design. Recirculating the air from the compressor discharge casing 120 may raise the inlet temperature of the compressor 110, reduce core airflow, and improve surge margin so as to enable operation at lower angles for the inlet guide vanes 150.

FIG. 2 shows a compressor recirculation configuration 175. In this configuration 175, the inlet bleed heat line 160 is connected directly to the compressor 110. Compressor air also can be extracted at any stage and then reintroduced to an earlier stage where needed. Recirculating the air from the compressor 110 thus may improve surge margins without an impact on the overall efficiency found in the use of the inlet bleed heat because such inlet bleed heat impacts the entire flow path of the compressor 110 (FIG. 1). The recycled air enables operation at lower angles for the inlet guide vanes 150 so as to reduce core airflow and raise combustion temperatures in the combustor 130.

FIG. 3 shows a schematic view of a compressor extraction configuration 180. This configuration 180 may include a number of compressor cooling lines 190. Each of the compressor cooling lines 190 may have a valve 200 positioned thereon. The valve 200 may be of conventional design. The compressor cooling lines 190 provide extractions from the compressor 110, bypassing the combustor 130, and cooling the turbine 140. This configuration 180 increases the extraction flow during turndown. The extraction flow may be reintroduced into the turbine 140 or into the exhaust path.

For example, a first compressor cooling line 190 may extend from a thirteenth stage of the compressor 110 to a stage two nozzle in the turbine 140 with a second compressor cooling line 190 extending from a ninth stage of the compressor 110 to a stage three nozzle in the turbine 140. Introduction into the exhaust path may be upstream or downstream of any type of exhaust temperature measurement location. The extractions may be taken from any stage of the compressor 110. There may be a common extraction location for cooling or there may be separate locations specifically for the purpose of bypassing air. The choice of location may depend on factors such as recycle efficiency, compressor operability, durability, and acoustics. Existing extraction locations may be used.

FIG. 4 shows a schematic view of a compressor discharge casing extraction configuration 210. This configuration 210 may include a compressor discharge casing cooling line 220 with a valve 230 thereon. The valve 230 may be of conventional design. The extraction may be taken from the same location as used in the inlet bleed heat line 160 or additional extractions may be used. The compressor discharge casing configuration 210 may improve compressor surge margin and may be able to increase extractions, inlet bleed heat, and a reduction in the minimum angles for the inlet guide vanes 150.

A number of these techniques may be employed depending on the overall configuration of the gas turbine 100. In fact, each method may be applicable for improving turndown performance. The selection of the methods and their operation and interaction will depend on the overall design of the gas turbine system 100 and related combustion technology. Specifically, the level of turndown improvement may depend upon the frame size of the gas turbine 100 and the particular combustion technology used.

For example, in a 7FA+e gas turbine with a dry-low-Nox 2.6 combustion system, the preferred configuration may include reducing the minimum angle of the inlet guide vanes 150, doubling the extraction flows, and adding an extraction from the compressor discharge casing 120 to bypass additional air to the exhaust. The 7FA+e gas turbine is available from the General Electric Company of Schenectady, N.Y. For a 9FB gas turbine with a similar combustion system, only reducing the minimum angle of the inlet guide vanes 150 with an increase in the isotherm may be required. The 9FB gas turbine also is available from the General Electric Company of Schenectady, N.Y. Other types of gas turbines may be used herein. By employing these various methods, emissions compliance may be maintained down to about thirty percent (30%) of the load. Other improvements may be possible.

It should be apparent that the foregoing relates only to the preferred embodiments of the present application and that numerous changes and modifications maybe made herein by one of ordinary skill in the art without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof. 

1. A gas turbine system for operation at low loads, comprising: a plurality of inlet guide vanes; a compressor; a turbine; and air movement means for maintaining an emission from the gas turbine system below a predetermined level.
 2. The gas turbine system of claim 1, wherein the air movement means comprises an inlet bleed heat line extending from the compressor to upstream of the plurality of inlet guide vanes.
 3. The gas turbine system of claim 1, wherein the compressor comprises a compressor discharge casing and wherein the air recirculation means comprises an inlet bleed heat line extending from the compressor discharge casing to upstream of the plurality of inlet guide vanes.
 4. The gas turbine system of claim 1, wherein the compressor comprises a plurality of stages and wherein the air movement means comprises a compressor recirculation line extending from a downstream compressor stage to an upstream compressor stage.
 5. The gas turbine system of claim 1, wherein the air movement means comprises a compressor cooling line extending from the compressor to the turbine.
 6. The gas turbine system of claim 5, wherein the compressor cooling lines comprises a plurality of compressor cooling lines.
 7. The gas turbine system of claim 1, wherein the air movement means comprises an extraction line extending from the compressor to downstream of the turbine.
 8. The gas turbine system of claim 1, wherein the compressor comprises a compressor discharge casing and wherein the air extraction means comprises an extraction line extending from the compressor discharge casing to downstream of the turbine.
 9. The gas turbine system of claim 1, wherein the plurality of inlet guide vanes comprises an angle of about thirty (30) to about fifty (50) degrees.
 10. The gas turbine system of claim 1, wherein the load thereon comprises about as low as about thirty percent (30%).
 11. A gas turbine for operation at low loads, comprising: a plurality of inlet guide vanes; a compressor; and air recirculation means to raise a temperature of an outlet air stream leaving the compressor.
 12. The gas turbine of claim 11, wherein the air recirculation means comprises an inlet bleed heat line extending from the compressor to upstream of the plurality of inlet guide vanes.
 13. The gas turbine of claim 11, wherein the compressor comprises a compressor discharge casing and wherein the air recirculation means comprises an inlet bleed heat line extending from the compressor discharge casing to upstream of the plurality of inlet guide vanes.
 14. The gas turbine of claim 11, wherein the compressor comprises a plurality of stages and wherein the air recirculation means comprises a compressor recirculation line extending from a downstream compressor stage to an upstream compressor stage.
 15. The gas turbine of claim 11, wherein the plurality of inlet guide vanes comprises an angle of about thirty (30) to about fifty (50) degrees.
 16. A gas turbine system for operation at low loads, comprising: a plurality of inlet guide vanes; a compressor; a turbine; and air extraction means to extract air from the compressor.
 17. The gas turbine system of claim 16, wherein the air extraction means comprises a compressor cooling line extending from the compressor to the turbine.
 18. The gas turbine system of claim 17, wherein the compressor cooling lines comprises a plurality of compressor cooling lines.
 19. The gas turbine system of claim 16, wherein the air extraction means comprises an extraction line extending from the compressor to downstream of the turbine.
 20. The gas turbine system of claim 16, wherein the compressor comprises a compressor discharge casing and wherein the air extraction means comprises an extraction line extending from the compressor discharge casing to downstream of the turbine. 